Oil sand deposits such as those found in the Athabasca Region of Alberta, Canada, contain a significant amount of heavy oil or bitumen. One recovery method that has been successful in extracting the heavy oil or bitumen from oil sand is commonly referred to as the hot water process and involves the liberation of the bitumen from the sand by forming oil sand slurry with hot water and separating the bitumen by froth flotation to form a bituminous froth. The bitumen present in the froth is then concentrated by diluting it with a solvent such as naphtha after which the diluted froth is centrifuged to remove substantially all of the water and mineral solids. Naphtha is then removed and the bitumen is ready for further upgrading to produce a synthetic crude oil.
Bitumen is a complex and viscous mixture of large or heavy hydrocarbon molecules which contain a significant amount of sulfur, nitrogen and oxygen. In order for bitumen to be processed in refineries, it must first be broken up into smaller hydrocarbon molecules (synthetic crude oil). Unlike the more useful smaller hydrocarbon molecules, bitumen is carbon rich and hydrogen poor. Thus, upgrading of bitumen to synthetic crude oil generally involves removing some carbon while adding additional hydrogen to make more valuable hydrocarbon products. This is generally done using four main processes: coking, which removes carbon and breaks large bitumen molecules into smaller parts; distillation, which sorts mixtures of hydrocarbon molecules into their components; catalytic conversions, which help transform hydrocarbons into more valuable forms; and hydrotreating, which is used to help remove sulfur and nitrogen and add hydrogen to molecules. The synthetic crude oil end product can then be further refined into jet fuels, gasoline and other petroleum products.
As mentioned, a useful process for upgrading bitumen is delayed or fluid coking. With fluid coking, the bitumen feedstock is introduced into a fluid coker reactor containing a fluidized bed of hot solids, preferably coke, and is distributed uniformly over the surfaces of the coke particles where it is cracked to vapors and to carbonaceous material which is deposited onto the particles. The vapors pass through cyclones which remove most of the entrained coke particles. The vapor is then discharged into a scrubbing zone where remaining coke particles are removed and the products are cooled to condense heavy liquids.
The coke particles in the coking zone flow downwardly to a stripping zone at the base of the coker reactor where a stripping gas, such as steam, is used to remove interstitial product vapors from, or between, the coke particles, as well as some adsorbed liquids from the coke particles. The coke particles are then removed to a burner where sufficient air is injected for burning at least a portion of the coke and heating the remainder sufficiently to satisfy the heat requirements of the coking zone where the unburned hot coke is recycled. Net coke, above that consumed in the burner, is withdrawn as product coke.
Coking produces a large amount of “sour water”, so called because of the large amount (e.g., between 0.3 and 10.4 wt %) of hydrogen sulfide (H2S) present therein. Also present in the sour water is a large amount (e.g., between 0.3 and 6.0 wt %) of ammonia (NH3). Another process for upgrading bitumen is catalytic cracking, which also produces sour water having significant quantities of H2S and NH3. Catalytic cracking involves the use of catalytic crackers operated at moderately-high temperatures (e.g., 400-500° C.), where a catalyst such as a zeolite catalyst is added to aid in “cracking” or splitting the large hydrocarbon molecules into smaller hydrocarbon molecules. It would be desirable to be able to recover the NH3 present in either coking sour water or catalytic cracking sour water, as NH3 is a valuable and useful product.
For example, during typical fluid coking operations, fuel gas produced in the coker burner is typically treated in a CO burner. However, the flue gas that is produced in the CO burner contains high levels of SO2 and thus it is undesirable to release the flue gas directly into the atmosphere without addressing the high levels of SO2 first. One process that may be used to remove SO2 from flue gas is flue gas desulfurization, which process uses anhydrous or aqueous ammonia which reacts with the SO2 to produce ammonium sulfate (see, for example, Canadian Patent No. 2,343,640, U.S. Pat. No. 4,690,807 and U.S. Pat. No. 5,362,458). The ammonium sulfate so produced can then be used as a fertilizer. Thus, flue gas desulfurization not only removes the SO2 present in the flue gas but also produces a valuable byproduct, namely, ammonium sulfate.
However, significant quantities of NH3 are needed in flue gas desulfurization, which can prove to be very costly. Thus, it would be desirable to recover NH3 from sour water streams produced during bitumen upgrading to synthetic crude that is of a sufficient quality so that it could be used in such a process. It is understood, however, that the NH3 recovered from sour water streams could also be used directly to make other useful products such as fertilizers and the like.